(i) Field of the Invention
The present invention relates to a screening method for selectively eliminating a laminated ceramic capacitor having a defect in an initial internal structure or a defect which may possibly reduce duration of life during a period of service.
(ii) Description of the Related Art
With a reduction in size and weight and an increase in performance of electronic devices, a reduction in size and an increase in capacity of laminated ceramic capacitors have also considerably advanced, a dielectric material layer thickness per layer has become very small, and a slight structural defect in the laminated dielectric material has become a factor which deteriorates the reliability of the laminated ceramic capacitor.
Thus, in the laminated ceramic capacitor, in order to selectively eliminate a product having an internal structural defect or a product which may possibly shorten a duration of line during a period of service, there is carried out a screening method by which a direct-current voltage which is several-fold or several-ten-fold of a rated voltage, e.g., a voltage which is not less than DC 300 V is applied, and a direct-current insulation resistance (which will be referred to as an IR hereinafter) is measured.
When executing screening, characteristics and an electrode structure of a dielectric material constituting a laminated ceramic capacitor must be taken into consideration, and the material characteristics and the electrode structure must be prevented from being adversely affected by screening. Further, considering that a product which may possibly shorten duration of life during a period of service must be selectively eliminated in advance, bearing a reliability evaluation is also required.
First, giving a consideration on a dielectric material with respect to the above-described condition which must be satisfied in screening, this type of dielectric material involves electrostriction. If a direct-current voltage which is not less than 300 V is applied between terminal electrodes of a multiplayer ceramic capacitor, the electrostriction may be capable of generating a crack or the like even in a high-quality product which does not have an internal structural defect. The crack can be a factor which provokes a sudden deterioration in the IR in a mounted state.
Setting an application voltage to a voltage level at which an electrostriction crack or the like is not generated can avoid the above-described problem. However, this result in insufficient screening for selecting a high-quality laminated ceramic capacitor.
A patent reference 1 (Japanese Patent Application Laid-open No. 1997-205037) discloses a technique which measures an IR in a state that a laminated ceramic capacitor is heated and a product having an abnormal measured resistance is selectively eliminated as a characteristic defective in place of just applying a direct-current voltage which is not less than a rated voltage to a laminated ceramic capacitor. However, although the patent reference 1 proposes that heating is effected under a temperature condition of 50 to 150° C., an embodiment only discloses a heating condition of 85° C. Under this temperature condition, occurrence of a crack involved by the electrostriction cannot be prevented.
A patent reference 2 (Japanese Patent Application Laid-open No. 2000-150328) discloses a screening method which applies a direct-current voltage which is not less than 300 V is applied as a withstand voltage test and measures an IR in a state that a laminated ceramic capacitor is heated at a temperature which is at least 150° C. or above as means for solving the problem in the patent reference 1.
However, although there is a description that an upper limit of the heating temperature is 200° C., this upper limit value is selected based on a relationship with a measuring instrument or the like. This patent reference 2 does not disclose a temperature condition which should be satisfied when an electrode structure of the laminated ceramic capacitor is taken into consideration.
Moreover, the patent reference 2 just discloses the screening method about a relaxa-based laminated ceramic capacitor, and it does not disclose a screening condition which should be satisfied about a laminated ceramic capacitor having other characteristics, e.g., a laminated ceramic capacitor having B characteristic, F characteristics or X7R characteristics. Additionally, it does not mention about a reliability evaluation.
A patent reference 3 (Japanese Patent Application Laid-open No. 2000-260653) describes a point that a capacitor is heated to a temperature which is not less than a Curie temperature of a dielectric material layer and not less than 125° C., and an IR and an IR life are estimated and judged based on a phase angle of an impedance and a relationship between a frequency and the phase angle. This patent reference does not discloses a technique which directly measures an IR.